Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce olefins such as ethylene and/or propylene from a variety of hydrocarbon feedstocks. Ethylene and propylene are important commodity petrochemicals useful in many processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. This process is referred to as the oxygenate-to-olefin, or OTO, process. Typically, the preferred oxygenate for light olefin production is methanol. The process of converting methanol to olefins is called the methanol-to-olefins, or MTO, process.
There are numerous technologies available for producing oxygenates, and particularly methanol, including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. The most common process for producing methanol is a two-step process of converting natural gas to synthesis gas. Then, synthesis gas is converted to methanol.
Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Synthesis gas production processes are well known, and include conventional steam reforming, autothermal reforming or a combination thereof.
Synthesis gas is then processed into methanol. Specifically, the components of synthesis gas (i.e., hydrogen, carbon monoxide and/or carbon dioxide) are catalytically reacted in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one process, methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor.
The methanol is then converted to olefin product in a methanol-to-olefins process. The methanol-to-olefins reaction is highly exothermic and produces a large amount of water. Water comprises about half of the total weight of the output stream of the reactor or effluent stream. Consequently, the water must be removed by condensation in a quench device to isolate the olefin product. The use of a quench device is one way to do this.
The effluent stream of an oxygenate-to-olefin reactor also contains byproducts including oxygenate byproducts such as organic acids, aldehydes, higher alcohols, and/or ketones. Carbon dioxide is also a byproduct of the oxygenate-to-olefin reaction. Furthermore, depending upon operating conditions, unreacted methanol is likely to be present in the effluent of the oxygenate-to-olefin reaction. Also, catalyst fines or particles may be present in the effluent of the oxygenates-to-olefins reaction.
Fouling within the quench process itself results in a decrease in efficiency of the overall oxygenates-to-olefins process and the quench process. Mitigation of the fouling would increase the efficiency of the process, allowing greater time intervals between shut downs. Further, partial neutralization of the organic acids and reduction of corrosion of the equipment are desirable.
U.S. Pat. Nos. 6,482,998 and 6,121,504 describe an oxygenate-to-olefin process that includes a quench tower for removal of water produced in the oxygenate-to-olefin reactor. Unreacted oxygenate feed (typically methanol) that is liquid under quenching conditions is removed from the quench tower as a heavy product fraction. The unreacted oxygenate feed is separated from water in the quench medium in a fractionation tower.
U.S. Pat. No. 6,403,854 and WO 03/104170 A1 describe a two-stage solids wash and quench for use with the oxygenate conversion process where catalyst fines are removed from the effluent stream through a first quench stage. The bottoms of the quench device include water, alcohols, ketones and neutralized organic acids that have a boiling point greater than water. The quench medium is a portion of the quench bottoms that is mixed with a neutralization stream and purified water stream. Therefore acids such as formic acid, acetic acid, butyric acid and propanoic acid can be neutralized. The neutralization material can be caustic, amines or ammonia.
U.S. Pat. No. 6,459,009 describes a two-stage quench tower process for removing impurities from a superheated reactor effluent stream withdrawn from an oxygenate conversion complex. The patent further describes the use of a neutralization material to neutralize any organic acid present in the effluent stream.
U.S. Patent Publication No. 2005/0234281 A1 describes a process for the catalytic conversion of a feedstream containing an oxygenate to light olefins using a fluidized conversion zone and a relatively expensive fluidized catalyst containing an ELAPO molecular sieve with recovery and recycle of contaminating catalyst particles from the product effluent stream withdrawn from the conversion zone.
U.S. Patent Publication No. 2006/0149111 A1 describes a process for converting oxygenate to olefins from a fluidized bed reactor which comprises removal of catalyst fines from a quenched vaporous effluent by contacting with a liquid low in catalyst fines content, e.g., oxygenate feedstock, or byproduct water from the oxygenates to olefins conversion which is stripped and/or filtered. The process typically comprises: contacting a feedstock comprising oxygenate with a catalyst comprising a molecular sieve under conditions effective to produce a deactivated catalyst having carbonaceous deposits and a product comprising the olefins; separating the deactivated catalyst from the product to provide a separated vaporous product which contains catalyst fines; quenching the separated vaporous product with a liquid medium containing water and catalyst fines, in an amount sufficient for forming a light product fraction comprising light olefins and catalyst fines and a heavy product fraction comprising water, heavier hydrocarbons and catalyst fines; treating the light product fraction by contacting with a liquid substantially free of catalyst fines to provide a light product fraction having reduced catalyst fines content and a liquid fraction of increased fines content; compressing the light product fraction having reduced catalyst fines content; and recovering the light olefins from the compressed light product fraction.
Notwithstanding the improvements in technology relating to the removal of impurities from olefin streams, further improvements in separating condensed water and entrained solids from an olefin stream are desired. In particular, such systems are sought where fouling is significantly reduced or eliminated.